Switching losses during dc-to-ac power conversion in conventional inverters are highly undesirable. Switching losses can occur for a variety of reasons, including simultaneous voltage across the switch and current through the switch during turn-on. Switching losses may also occur as a result of reverse recovery current of a diode opposite the switch. Such switching losses increase the need for component shielding due to EMI noise, and require greater heat protection due to increased power consumption. These undesirable effects of switching losses increase costs and reduce efficiency, thus reducing the acceptance of such conventional inverter techniques to industries. A resonant inverter circuit which achieves lossless, or zero voltage switching (soft switching) is therefore needed for efficiency reasons.
Soft switching snubber inverters of the present type normally use the resonant technique to reduce or eliminate the switching loss so that the frequency can be increased. High frequency soft switching improves inverter performance by elimination of acoustic noises and electromagnetic interferences, reduction of torque ripple, and improvement of efficiency. However, the resonant circuit generally produces either over-voltage or over-current conditions during switching. A typical resonant dc link inverter driving a three-phase ac motor can experience a peak dc bus voltage of twice the supply voltage in motoring mode operation, and the overshoot voltage can be more than three times the supply voltage in the regenerative mode. These inverters are also restricted to switching only at the zero voltage crossing to avoid short circuit destruction so that if zero voltage is never actually achieved then the device cannot switch. Other resonant inverters, such as a typical clamped mode resonant pole inverter, can experience overshoot current of more than twice the rated load current.
A resonant snubber inverter circuit currently exists which reduces over-voltage and over-current problems in the circuit. FIG. 1 shows such a three-phase resonant snubber circuit 6 which reduces over-voltage and over-current problems in resonant inverter circuits 8 without requiring that switching only occur at the zero voltage crossing. For each phase, this resonant snubber circuit 6 utilizes two anti-parallel auxiliary switches 14 & 16, 18 & 20, and 22 & 24 to control the resonance for the phase with each auxiliary switch pair being connected to the center tap of two energy storage capacitors 10 and 12 to provide the snubber voltage, a blocking diode 15, 17, 19, 21, 23, and 25 connected in series with each auxiliary switch, two high frequency resonant capacitors 44 & 46, 48 & 50, and 52 & 54, and a resonant inductor 26, 28, and 30. The main switches 32, 34, 36, 38, 40, and 42 control the current flowing to the motor 56. The six auxiliary switches 14-24, in series with their respective reverse blocking diodes 15-25, control snubbing during commutation of the main switches 32-42.
During commutation, resonant snubber inverters of the present type provide soft turn-on of the main switches without means external to the circuit. Without resonant snubbers, when turning on the main switches 32-42, the energy stored in the resonant capacitors 44-54 charges back to the main switches 32-42, and the reverse recovery current caused by the free-wheeling diodes 45-55 adds in, resulting in a large spiky current. The resonant snubber circuit 6 in FIG. 1 solves the turn-on over-current problem. For example, when the motor phase A current is positive and is flowing through the free-wheeling diode 47 which is anti-paralleled against main switch 34, before turning on main switch 32 when main switch 34 is still on, auxiliary switch 16 is turned on and a current flows through the resonant inductor 26 and main switch 34. When this current exceeds the load current, a resonance occurs between the resonant inductor 26 and the resonant capacitors 44 and 46. The resonant current diverts the current from the diode 47 against switch 34 to the diode 45 against switch 32, resulting in zero voltage across main switch 32. Zero voltage switching can then be achieved.
Although the FIG. 1 resonant snubber inverter allows the main switches to turn on at zero voltage across the main devices, the quantity of components required for this circuit makes its application less desirable.